An example of the prior art is the technique disclosed in JP-A-5-189027 (Patent Reference 1).
The Patent Reference 1 discloses a technique in which in diagnosing signal abnormality, for a predetermined time interval (sampling period) in a normal running state of the signal to be diagnosed, the maximum width of a signal change or the maximum value of a signal changing rate is stored as that in the normal running sate and supplied with a margin to set for a threshold value of signal abnormality diagnosis, and the signal abnormality diagnosis is done on the basis of the threshold value.
Patent Reference 1: JP-A-5-189027